You define which scripts
gab
will run on (i.e. showthread, search, etc...). Each script then runs 1 or more billboards. A billboard can have 1 or more ads (though they have to be the same type) and they can be limited to a particular category.

Its a bit complex, but what you are trying to do is easy to achieve by:

Creating 2 billboards (vert and horizontal). Limit the ads to be pulled from corresponding categories (i.e. cateogries that hold only vertical ads or horizontal ads).
Then assign those billboards accordingly in your script.

Hope that didnt muddle it up too much

GAB

GAB is a highly configurable banner system that allows you to customize banners for vBulletin easily. Determine which usergroups see banners, which ones can disable them, which pages have banners and where. GAB is the one stop shop for banner advertising.